Make ptx cache settable by environment variable (#2304)

mlx/backend/cuda/jit_module.cpp

@@ -37,7 +37,8 @@ void check_cu_error(const char* name, CUresult err) {
 }
 
 // Return the location of the CUDA toolkit.
-const char* cuda_home() {
+const std::string& cuda_home() {
+  static std::string home = []() -> std::string {
     const char* home = std::getenv("CUDA_HOME");
     if (home) {
       return home;
@@ -54,19 +55,28 @@ const char* cuda_home() {
 #endif
     throw std::runtime_error(
         "Environment variable CUDA_HOME or CUDA_PATH is not set.");
+  }();
+  return home;
 }
 
 // Get the cache directory for storing compiled results.
-bool get_ptx_cache_dir(std::filesystem::path* result) {
+const std::filesystem::path& ptx_cache_dir() {
-  auto path = std::filesystem::temp_directory_path() / "mlx" / "ptx";
+  static std::filesystem::path cache = []() -> std::filesystem::path {
-  if (!std::filesystem::is_directory(path)) {
+    std::filesystem::path cache;
+    if (auto c = std::getenv("MLX_PTX_CACHE"); c) {
+      cache = c;
+    } else {
+      cache = std::filesystem::temp_directory_path() / "mlx" / "ptx";
+    }
+    if (!std::filesystem::exists(cache)) {
       std::error_code error;
-    if (!std::filesystem::create_directories(path, error)) {
+      if (!std::filesystem::create_directories(cache, error)) {
-      return false;
+        return std::filesystem::path();
       }
     }
-  *result = path;
+    return cache;
-  return true;
+  }();
+  return cache;
 }
 
 // Try to read the cached |ptx| and |ptx_kernels| from |cache_dir|.
@@ -75,6 +85,10 @@ bool read_cached_ptx(
     const std::string& module_name,
     std::vector<char>* ptx,
     std::vector<std::pair<std::string, std::string>>* ptx_kernels) {
+  if (cache_dir.empty()) {
+    return false;
+  }
+
   auto ptx_path = cache_dir / (module_name + ".ptx");
   std::error_code error;
   auto ptx_size = std::filesystem::file_size(ptx_path, error);
@@ -105,6 +119,10 @@ void write_cached_ptx(
     const std::string& module_name,
     const std::vector<char>& ptx,
     const std::vector<std::pair<std::string, std::string>>& ptx_kernels) {
+  if (cache_dir.empty()) {
+    return;
+  }
+
   std::ofstream ptx_file(cache_dir / (module_name + ".ptx"), std::ios::binary);
   if (!ptx.empty()) {
     ptx_file.write(&ptx.front(), ptx.size());
@@ -184,11 +202,9 @@ JitModule::JitModule(
     const std::string& module_name,
     const KernelBuilder& builder) {
   // Check cache.
-  std::filesystem::path cache_dir;
   std::vector<char> ptx;
   std::vector<std::pair<std::string, std::string>> ptx_kernels;
-  if (!get_ptx_cache_dir(&cache_dir) ||
+  if (!read_cached_ptx(ptx_cache_dir(), module_name, &ptx, &ptx_kernels)) {
-      !read_cached_ptx(cache_dir, module_name, &ptx, &ptx_kernels)) {
     // Create program.
     auto [source_code, kernel_names] = builder();
     nvrtcProgram prog;
@@ -246,7 +262,7 @@ JitModule::JitModule(
     } else {
       CHECK_NVRTC_ERROR(nvrtcGetPTX(prog, ptx.data()));
     }
-    write_cached_ptx(cache_dir, module_name, ptx, ptx_kernels);
+    write_cached_ptx(ptx_cache_dir(), module_name, ptx, ptx_kernels);
   }
 
   // Load module.